Seroprevalence of influenza viruses in Shandong,Northern China during the COVID-19 pandemic

Nonpharmaceutical interventions(NPIs)have been commonly deployed to prevent and control the spread of the coronavirus disease 2019(COVID-19),resulting in a worldwide decline in influenza prevalence.However,the influenza risk in China warrants cautious assessment.We conducted a cross-sectional,sero-epidemiological study in Shandong Province,Northern China in mid-2021.Hemagglutination inhibition was performed to test antibodies against four influenza vaccine strains.A combination of descriptive and meta-analyses was adopted to compare the seroprevalence of influenza antibodies before and during the COVID-19 pandemic.The overall seroprevalence values against A/H1N1pdm09,A/H3N2,B/Victoria,and B/Yamagata were 17.8％(95％CI 16.2％-19.5％),23.5％(95％CI 21.7％-25.4％),7.6％(95％CI 6.6％-8.7％),and 15.0(95％CI 13.5％-16.5％),respectively,in the study period.The overall vaccination rate was extremely low(2.6％).Our results revealed that antibody titers in vaccinated participants were significantly higher than those in unvaccinated individuals(P＜0.001).Notably,the meta-analysis showed that antibodies against A/H1N1pdm09 and A/H3N2 were significantly low in adults after the COVID-19 pandemic(P＜0.01).Increasing vaccination rates and maintaining NPIs are recommended to prevent an elevated influenza risk in China.